1. Field of the Invention
This invention relates generally to flywheel energy storage systems as may be suitable for stationary and mobile energy storage applications. More specifically, this invention relates to flywheel systems that use wire-wound rotors.
2. Description of the Related Art
A number of materials have been used to construct rotors for energy storage flywheels. The selection of rotor material dictates the configuration of the system and sets a floor below which rotor cost may not be reduced.
Rotors in commercially available flywheel systems have been constructed from forged steel and from carbon and glass fiber composites. However, forged steel rotors suffer from low tensile strength compared to alternatives, thus limiting the amount of energy they can store.
Composite rotors also have their own significant drawbacks. Composite rotors are typically fabricated by laying down thousands of microfibers with wet epoxy, where the rotor reaches its full strength once the epoxy has fully cured. However, in order to store appreciable energy, flywheel rotors are typically much thicker than other cylindrical composite structures, such as pressure vessels. As a result, the fabrication process is more complicated compared to thinner structures and can become slow and expensive. For example, sufficient time and care must be taken to allow the epoxy to set during winding, to prevent the microfibers from unduly compressing the underlying layers, and to allow sufficient heat release during the epoxy curing process. Any inattention can result in structural weaknesses in the rotor. Some of these problems can be reduced by making the composite layer thinner, but then either more mass must be added in the axial direction or the rotor must be operated at a higher speed in order to store the same amount of energy. Composites are also less dense than metals, so composite rotors must be spun at higher speeds in order to store the same amount of energy as a comparably-sized metal rotor.
Thus, there is a need for improved rotor designs.